a. Field of the Invention
The invention relates to a method and a device for recognition of a focussing status of a light beam incident on an information carrier.
b. Description of the Prior Art
The method according to the invention for recognition of a focussing status can be employed in a corresponding recording or replay device for contactless scanning of optical information carriers. In a recording or replay device of this type, an information carrier, for example a CD, is illuminated by a scanning light beam. A light beam reflected by the information carrier carries the information stored on the information carrier. In the case of CDs, by far the most common optical information carrier, usually elongate indentations are made on a plane reflecting face. These indentations, also referred to as pits, form a track in the form of a spiral or concentric circles. The reflected light beam has a different intensity, depending on whether the scanning light beam is incident on the CD at an indentation or at a plane location.
In order to obtain an unambiguous signal, the scanning light beam needs to be focussed very well. On the one hand, the light spot on the information carrier face carrying the information should not be too large, so that neighbouring pits or tracks are not picked up at the same time, and on the other hand the light spot should not fall short of a particular minimum size. This minimum area is necessary, for example in the case of a CD, so that, for incidence on a pit, a sufficient area around the pit is also illuminated. The pit gap conventionally used in a CD then causes a drop in intensity, due to destructive interference, in the reflected light beam.
Conventional optical scanning systems are therefore provided with an autofocus system for automatically setting or correcting the focussing of the scanning light beam. A known design for recognizing the focussing status of a light beam incident on an information carrier uses astigmatism.
This design is explained with the aid of a conventional scanning system with reference to FIG. 6. In FIG. 6, a scanning light beam 14 is emitted by a light source 12, in particular a semiconductor laser. The scanning light beam 14 passes through a beam splitter 29 and is focussed using a converging lens 13 in such a way that the focus falls on the information track 30 of an information carrier 11. A light beam 15 reflected by the information carrier 11 carries the information read from the information track 30, passes again through the converging lens 13 and is reflected down onto a cylindrical lens 31 by the beam splitter 29. In the reflected light beam 15, the cylindrical lens 31 produces artificial astigmatism, so that the reflected light beam 15 is converted into an irregular light beam. This light beam does not have a cross-section with symmetry of rotation about the propagation direction. It does not have a single focal point, but has two focal lines 33, 34 which are spaced apart and one of which extends in the x direction of the cylinder axis of the cylindrical lens 31 while the other extends in the y direction perpendicular thereto.
If the information carrier 11 moves in a direction in which the distance to the converging lens 13 becomes smaller, the cross-section of the light beam at point 32 becomes broader in the x direction. If, however, the information carrier 11 is moved away from the converging lens 13, the cross-section of the reflected light beam 15 at point 32 becomes broader in the y direction. This means that the so-called astigmatism and the concomitant change in the cross-section of the reflected light beam 15 can be used as a focussing error signal, in order to control or regulate the distance from the converging lens 13 to the information carrier 11 so that the pits in the information-carrying face of the information carrier 11 always lie within the focus of the scanning light beam. The change in the cross-section of the reflected light beam at point 32 can be scanned using a light-sensitive detector 18, in particular a photodiode.
DE 40 02 015 C2 discloses a further scanning system, in which the focussing status of the light beam incident on the information carrier can be recognized. In the scanning system presented there, a prism with parallelogram-shaped configuration is used instead of the cylindrical lens.
The use of a prism is proposed in the arrangement described there in order to achieve an optical system with reduced dimensions. On the one hand, in the optical system proposed there, the beam splitter can be omitted, since the prism itself constitutes one. On the other hand, the overall arrangement is made shorter by the reflected light beam being reflected to and fro repeatedly in the prism itself.
However, in order to ensure the beam-splitting function as well as the multiple reflection of the reflected light beam, elaborate alignment of the prism is necessary.
A likewise elaborate alignment is necessary when using a cylindrical lens. Furthermore, a cylindrical lens has the disadvantage that it requires a relatively high degree of outlay on production. In order to achieve a sufficiently large focal spot, the cylindrical lens is furthermore combined with a negative lens, i.e. a diverging lens.
The object of the invention is to provide a method for recognizing a focussing status by utilizing astigmatism, in which significant astigmatism can be achieved in the reflected light beam simply and cost-efficiently without elaborate alignment. A further intention is to provide a corresponding recording or replay device.